Tag: Linux

Virtual Loopback Filesystem: Building A Linux Filesystem From An Ordinary File

Posted on by admin

Unix / Linux has a great inbuilt simplicity to it where you can create file systems on any block set . Now .. a file is a block  set so technically and truly at it’s core a file system can be built on a file and mounted :

I need a 100MB partition for some tests :

Using ofcourse Google as my calculator {pfsssst !!! obviously .. DUH}. I tried 100MB / 512 bytes and got

(100 megabytes) / (512 bytes) = 204 800

 
[root@localhost admin]# dd if=/dev/zero of=disk-image count=204800
204800+0 records in
204800+0 records out
104857600 bytes (105 MB) copied, 41.8633 s, 2.5 MB/s
[root@localhost admin]#

This command leaves me with a 105MB file in which to create my etx3 fs like so :

[root@localhost admin]# ls -al disk-image 
-rw-r--r--. 1 root root 104857600 Jul 13 18:17 disk-image
[root@localhost admin]#

Once this file is created I will use the mkfs to format over the disk-image file:

[root@localhost admin]# /sbin/mkfs -t ext3 -q disk-image
disk-image is not a block special device.
Proceed anyway? (y,n) y
[root@localhost admin]#

Thus a filesystem now resides on my file named disk-image.

We can now use a loopback device to mount the disk-image anywhere we need it:

[root@localhost admin]# mkdir /mnt/test1
[root@localhost admin]# mount
/dev/sda2 on / type ext4 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
tmpfs on /dev/shm type tmpfs (rw,rootcontext="system_u:object_r:tmpfs_t:s0")
/dev/sda1 on /boot type ext4 (rw)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
[root@localhost admin]# mount disk-image /mnt/test1/
mount: /home/admin/disk-image is not a block device (maybe try `-o loop'?)
[root@localhost admin]# mount -o loop disk-image /mnt/test1/
[root@localhost admin]# cd /mnt/test1/
[root@localhost test1]# ls
lost+found
[root@localhost test1]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda2              18G  2.7G   14G  16% /
tmpfs                 504M  768K  503M   1% /dev/shm
/dev/sda1             291M   50M  226M  19% /boot
/home/admin/disk-image
                       97M  5.6M   87M   7% /mnt/test1

[root@localhost test1]# touch file1
[root@localhost test1]# mkdir -p top1/bottom1
[root@localhost test1]# tree
.
+-- file1
+-- lost+found
+-- top1
    +-- bottom1

3 directories, 1 file
[root@localhost test1]#

Thus a new file system is now running inside the file.

Backing up that file or moving it to another system would entail now quiescing any applications/scripts working with the file, unmounting the loopback fs system, doing your thing then replace all the cards on the deck 🙂

Tar a folder or entire system through ssh

Posted on by admin

We all had the problem of needing to backup a folder or an entire system from a machine before decommissioning or as a postfix backup solution only to find tarring aint gonna work cause you have very little space left. Also there are folders you want to avoid tarring since they contain logs or system virtual folders that you want to skip.

So you need to get a tar from a machine have very little space left and you need to pull all the files in a compressed fashion; the following command calls a backup as root through ssh using tar on the source machine and skips the folders you do not want :

ssh root@pollux "tar zcvf - --exclude=/proc/* --exclude=/dev/* --exclude=/sys/* --exclude=/var/logs/* /" > /root/backup/pollux.fullandfinal.$(date '+%Y-%m-%d-%k-%M').tar.gz

The result of the above command is a backup of the / compressed at source piped through console through ssh back to your local / backup repo machine in .tar.gz format leaving out the rubbish thanks to the :

- --exclude=/proc/* --exclude=/dev/* --exclude=/sys/* --exclude=/var/logs/*

section. Note the “– —” the first – indicates extended parameters and –except leaves out any match to the path.

There you go one command to back them all.

(this could be easily bash-ed and cron-ed to produce a decent makeshift backup tool.

To check out the contents of the tar file you can use

tar -tvzf <archive.tar.gz>

This will show you the contents and leave a fail exit code in a bash script.

 

The Linux Conundrum

Posted on by admin

A large company, was taking over our smaller company and they were on a trend to replace Linux and Java with MS Windows  ®  and ASP.NET.

When the CIO was asked why not go the other way since arguably our smaller company was more advanced put plainly his answer “Linux and Java guys are so hard to find! (and expensive). MS Windows ® guys are all over the place … ”

I liked the proposition that Linux guys are not easy to find, is this really so ..? (feel free to comment) GOOD !!  🙂

So now I know Linux/ Unix is niche, and better paid, but I cannot but ask myself the question why is this so. Is MS Windows ® so much easier or is Linux still growing into a user OS ? and why in the server business is ease of use given importance over customize-ability and tweak-ability.

Also is Linux in any deep way better that MS Windows ®. In my opinion the differences are more in the approach and the attitude of trust towards a single focal point i.e. MS in this case or on a community led by the benevolent dictator  Linus Torvalds . (By the way this is how he pronounces Linux.  [Linux])

I think there is a whole discussion behind this but money affairs aside how did we end up where we are with Linux being so popular and still perceived as difficult.

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